As products get smaller and smaller, there is greater demand for micro-electrical-mechanical systems (MEMS), micro-optical devices and photonic crystals. With this demand, there is an associated increased interest in micro- and nano-machining. There are numerous possible applications for MEMS. As a breakthrough technology, allowing unparalleled synergy between previously unrelated fields such as biology and microelectronics, many new MEMS applications have emerged and many more may emerge in the near future, expanding beyond those currently identified or known. Additional applications in quantum electric devices, micro-optical devices and photonic crystals are also emerging.
Here are a few applications of current interest:
Quantum Electrical Devices
Interest in ideas such as quantum computing have lead to the development of devices requiring increasing smaller dimensions, such as cellular automata and coupled quantum dot technologies. Resonant tunneling devices such as resonant tunneling diodes, which may utilize quantum effects of transmission electrons to increase the efficiency of microwave circuits, require particularly fine features.
Micro-Optics
The application of micro-machining techniques to optics has lead to numerous advances in optical fabrication such as gray scale technology. Gray scale technology allows for the creation of a wide variety of shapes allowing for the best optical performance achievable. Traditional binary optics rely on a “stair step” shaped approximation of the ideal surface shape. Gray scale can actually create that ideal shape. Curves, ramps, torroids, or any other shape is possible. Multi-function optics, microlens arrays, diffusers, beam splitters, and laser diode correctors may all benefit from the use of gray scale technology. These optical devices as well as others, including fine pitch gratings for shorter and shorter wavelength light, benefit from increased precision available using micro-machining. Optical MEMS devices including beam shapers, continuous membrane deformable mirrors, moving mirrors for tunable lasers, and scanning two axis tilt mirrors have also emerged due to progress in micro-machining technology.
Photonic Crystals
Photonic crystals represent an artificial form of optical material that may be used to create optical devices with unique properties. Photonic crystals have many optical properties that are analogous to the electrical properties of semiconductor crystals and, thus, may also allow the development of optical circuitry similar to present electrical semiconductor circuitry. The feature sizes used to form photonic crystals and the precise alignment requirements of these features complicate manufacture of these materials. Improved alignment techniques and reduced minimum feature size capabilities for micro-machining systems may lead to further developments in this area.
Biotechnology
MEMS technology has enabling new discoveries in science and engineering such as: polymerase chain reaction (PCR) microsystems for DNA amplification and identification; micro-machined scanning tunneling microscope (STM) probe tips; biochips for detection of hazardous chemical and biological agents; and microsystems for high-throughput drug screening and selection.
Communications
In addition to advances that may result from the use of resonant tunneling devices, high frequency circuits may benefit considerably from the advent of RF-MEMS technology. Electrical components such as inductors and tunable capacitors made using MEMS technology may perform significantly better compared to present integrated circuit counterparts. With the integration of such components, the performance of communication circuits may be improved, while the total circuit area, power consumption and cost may be reduced. In addition, a MEMS mechanical switch, as developed by several research groups, may be a key component with huge potential in various microwave circuits. The demonstrated samples of MEMS mechanical switches have quality factors much higher than anything previously available. Reliability, precise tuning, and packaging of RF-MEMS components are to be critical issues that need to be solved before they receive wider acceptance by the market.
Advances in micro-optics and the introduction of new optical devices using photonic crystals may also benefit communications technology.
Accelerometers
MEMS accelerometers are quickly replacing conventional accelerometers for crash air-bag deployment systems in automobiles. The conventional approach uses several bulky accelerometers made of discrete components mounted in the front of the car with separate electronics near the air-bag. MEMS technology has made it possible to integrate the accelerometer and electronics onto a single silicon chip at ⅕ to 1/10 of the cost of the conventional approach. These MEMS accelerometers are much smaller, more functional, lighter, and more reliable as well, compared to the conventional macro-scale accelerometer elements.
Micro-Circuitry
Reducing the size of electronic circuits is another area in which MEMS technology may affect many fields. As the density of components and connections increases in these microcircuits, the processing tolerances decrease.
In some applications, such as photonic crystals or fiber Bragg gratings, in order to make fabricated device function, there are stringent requirements not only on feature sizes (<1000 nm in some cases), but also on positioning accuracy of these nano features (such as ˜10 nm). Traditionally, both these feature size and positioning accuracy requirements have been difficult to meet with conventional laser micromachining methods. Thus, the micromachining of submicron features has been a domain predominated by electron-beam, ultraviolet beam, and X-ray lithographic machines, as well as focused ion beam machines. These high-cost techniques usually require stringent environmental conditions, such as high vacuum or clean room condition. Standard lithographic methods require a separate operation for generating multiple masks.
If a beam processing technique is used, this process requires the beam to be directed accurately at the desired location with a high degree of precision for proper processing. Only four currently available technologies (laser direct writing, focused ion beam writing, micro electric discharge machine, and photochemical etching) have this potential capability. Other techniques (for example ion beam milling) are only desirable for flat wafer processing. However, direct laser writing has additional advantages including: (1) operation in ambient air under optical illumination; (2) the capability of forming structures inside transparent materials; and (3) low materials dependence. Direct laser writing may also be used to expose photoresist as part of a lithographic technique without the need to pregenerate mask sets.
Typically, ultrafast lasers in the visible (dye laser) or IR range (the fundamental wavelength of Ti:Sapphire or Nd:YLF) have been used for laser machining applications. It is known that the minimum spot size of a focused laser beam is approximately 2.44 times the f# of the objective lens, times the peak wavelength of the laser, i.e. the spot size is proportional to the peak wavelength. Thus, in system where a visible or an IR laser is used for nanomachining, the spot size is undesirably large for forming submicron features, even if high numerical aperture (low f#) optics are used. For example, if a Ti: Sapphire laser having an 800 nm peak wavelength and optics with an f# of 1 at 800 nm are used, the minimum size beam spot has a diameter of 1952 nm.
Even with this disadvantage, in late 1999 and early 2000, the capability of frequency doubled Ti: Sapphire femtosecond laser with a peak wavelength of 387 nm to machine ˜200 nm air holes in plain Si-on-SiO2 substrate was demonstrated. This submicron feature was achieved by controlling the fluence of the beam spot such that ablation only occurs near the intensity peak of the laser beam spot. However, this technique has a number of drawbacks for precise nanomachining, since the center of area actually machined may be somewhat offset from the center of the intensity profile. This uncertainty of the machining center may be induced by defects or imperfections of the material being processed, or may be due to slight pulse-to-pulse variations in the beam profile. Additionally, as the ratio of the machined area to the beam spot decrease, reducing any fluence fluctuations between pulses becomes increasingly critical.
UV lasers, having a peak wavelength <400 nm (mainly excimer lasers and frequency-converted YAG and YLF lasers), have been shown to provide superior surface finish compare to lasers in visible or IR range. In addition, since the minimum spot size is proportional to wavelength, UV lasers may be focused to a smaller spot size. Because most of these lasers have a pulse duration of >1 ns, they may cause undesirable heat effected zones to develop in the surrounding material during machining. Thus, these lasers may be undesirable for many nanomachining applications.